Method of preparing nitrating acids



June 6, 1955 J. REVALLIER 2,

METHOD OF PREPARING NITRATING ACIDS Filed April 21, 1953 INVENTOR L EONARDUS J. REl ALL/ER ATTORNEYS United States Patent METHOD OF PREPARING NITRATIN G ACIDS Leonardus J. Revallier, Geleen, Netherlands, assignor to Stamicarbon N. V., Heerlen, Netherlands Application April 21, 1953, Serial No. 350,189 Claims priority, application Netherlands April 25, 1952 Claims. (Cl. 23-114) Thepresent invention relates to a method of preparing nitrating acids, i. e., mixtures of nitric acid and sulfuric acid or oleum which are suitable for effecting nitrating reactions. Preferably, these nitrating acids should con tain an excess of sulfur trioxide not bound to water in addition to the nitric acid and the sulfuric acid.

A simple and direct method of preparing nitrating acids is described in British patent specification 524,531. Specifically, this British patent describes a method which involves the absorption of S03 or SOs-containing gases in nitric acid or mixed acid already available, preferably by passing these materials through a scrubber in countercurrent relationship, while cooling suificiently to prevent nitric acid vapors from escaping.

The preparation of nitrating acids as described in British Patent 524,531 suffers from several disadvantages. In the first place, very intensive cooling is necessary, because considerable heat is liberated by the reaction between $03 and HNOs. The desired cooling may be effected by allowing relatively little S03 to be absorbed per unit of volume of absorption liquid and repeating the operation to achieve the desired absorption. This, of course, is not particularly desirable.

Another disadvantage of the procedure described in British Patent 524,531 is that the desired nitrating acids can only be prepared if virtually pure S03 is available. Thus, contact gas, which contains nitrogen and oxygen, in addition to 6% to 8% by volume of S03, is not suited for the preparation of nitrating acids in the process of British Patent 524,531. This is due to the fact that the nitrogen and oxygen present in the contact gas and which are not absorbed by the absorption liquid containing nitric acid, entrain nitric acid'vapors from the absorption liquid thereby causing serious losses of nitric acid even when cooling is employed.

Although not specifically disclosed in the above mentioned British patent, it is possible to absorb S03 in a circulating mixed acid from contact gas accordingto the method described therein. When this is done, the circulating mixed acid must have a very low HNOa vapor pressure. This is only the case with mixed acids which contain, in addition to free S03, relatively littleHN03, e. g., less than 20% of HN03. Such a mixed acid has too high an S03 content in proportion to its HNOs content to permit its use as a nitrating acid because such a high S03 content causes side reactions, such as carbonization or sulfonation, to occur.

If it should still be desired to prepare nitrating acid from mixed acids containing relatively little HN03, by addition of concentrated HN03 to increase the proportion of HNOa to S03, the difiiculty is encountered that great quantities of heatare released, so that, in spite of proper cooling, loss of nitric acid due to decomposition will occur.

Itv has now been found that such free S03 containing nitrating acids having a content of, for example, 40% or more of HNOa can be prepared without difiiculty with respect to cooling requirements and losses of nitric acid through decomposition. Accordingly, the provision of .2 a process for preparing nitrating acids with the above mentioned advantages is the principal object of the invention although other objects, including novel improvements in the production of (N0z+) (HSaO'z), are also contemplated.

Broadly stated, the production of nitrating acids is carried out, according to the present invention, by a process which comprises dissolving solid mixed acid of the composition (N02+) (HSzOT') in concentrated nitric acid.

The success of the invention is due, at least to a large extent, to the discovery that in dissolving solid crystals of the product (N02+) (HSZO'F) in nitric acid, the heat of reaction is, to a large extent, compensated'by the heat consumed by the crystals when they melt during dissolution.

The fact that these solid crystals can be dissolved in nitric acid without dilficulty as far as the cooling and decomposition of nitric acid are concerned can be utilized with advantage in preparing the above mentioned solid crystals of the composition (N02+) (HSzOr).

As far back as 1912 W. Schultze described in United States specification No. 1,047,576 the preparation of the crystalline compound, which he indicated by the formula N2O5(S03)4.H20. However, recent roentgenographic investigations have shown that the compound is composed of NO2+ and HS207- ions. Accordingly, the formula (N02+) (HSZO'F) should be preferred to the old formula N20s(S0a)4.H2O given the crystalline compound described in the above mentioned United States Patent No. 1,047,576.

The Schultze method of preparation consists in the absorption of S03 gas in a liquid mixture of concentrated nitric acid and oleum in which the molar ratio between S03 and N205 is about 4, the liquid mixture being maintained at a temperature of over C. during the absorption. At the same time a flow of concentrated nitric acid is supplied to the absorption liquid as N205 makeup in such an amount that per unit'of time the ratio .between the quantities of S03 and N205 introduced, expressed in grammoles, is maintained at a value of about 4. Hot concentrated liquid is removed continuously, from which, after cooling, the solid product crystallizes out and is separated from the mother liquor by suction filtration.

The Schultze method of preparation has the serious disadvantage that the addition of concentrated nitric acid to the absorption liquid, which is kept at a temperature of over 100 0., causes loss of nitric acid due to partial decomposition thereof.

Presumably, this is the reason why the Schultze process has never been carried out on a technical scale, no commercial uses having been found for the crystalline product (N0z+) (HSaOz) to the present time, in spite of its favorable properties such as high thermal stability and low vapor pressure.

I have discovered that addition of N205 make-up .to the SOs-absorbing liquid can be accomplished without serious loss of nitric acid due to decomposition thereof by continuously supplying. N205-containing liquid to the absorbing liquid, which is kept at over 100 C., said N205- containing liquid being a nitrating acid and preferably consisting essentially apart from 0% to 2% of water, of a mixture of 35 to 50 parts by weight of S05 and 65 to 50 parts by weight of. HNOs. The utilization of such N205 make-up liquids has the great advantage that there is practically no decomposition of nitric acid .at temperatures of from 100-120 C. It was found that when these NzOs-make-up liquids were maintained at temperatures of from 100-l20 C. for half an hour, the nitritecontent (a measure of the degree of decomposition) only rose from 0.4 to 1.2% of N203. Concentrated nitric acid could not be heated to this extent, owing to decomposition.

Preferably, the absorbing liquid is continuously recirculated through the absorbing tower and the N205- make-up liquid is prepared by dissolving part of a previous production of the crystalline compound in nitric acid.

Also, the HN03 vapor pressure of the make-up liquids in question is relatively low, as a result of the fact that, in contradistinction to nitric acid, the HNO3 in the make-up liquid is, to a great extent, present in a. stable form (due to the formation of nitronium ions according to the equation HNO3+SO3SNO2++HSO4 Whereas 100% HNOs boils at 83 C., the HNOs vapor pressure of a liquid consisting of 35 to 50 parts of S03 and 65 to 50 parts of HNOs is only 0.1 to 0.3 atm. at 100 C. This low HNOs vapor pressure greatly reduces HNOs losses during the preparation of the solid nitrating acid.

A preferred method of preparing liquid and solid nitrating acids according to the invention will be explained with the help of the accompanying drawing in which the process of the invention and the equipment employed is represented diagrammatically.

In the drawing, 1 represents an absorption column filled with packing material, through which the absorption liquid is circulated continuously via a buffer tank 2 and a pump 3. The contents of the buffer tank may be kept at a temperature of ll-l20 C. by means of suitable heating appliances (not shown), e. g., electrically heated steam coils.

At the bottom of the column is the supply conduit 4 for the SOs-containing gas; at the top of the column is a gas delivery conduit 5 for the removal of the non-absorbed gases.

The buffer tank 2 is provided with a stirrer 12., a supply conduit for the NzOs-make-up liquid 13 and a discharge conduit 6 for the product formed by the reaction between $03 and NzOs-make-up liquid, which product is still liquid at a temperature of 110-l20 C.

The discharge conduit 6 debouches into a vessel 14 provided with a drum cooler 7. The solid product crystallizes upon the drum cooler from which it is removed in the form of crystal flakes by means of a doctor blade 15.

Part of the crystal mass produced is introduced into a reservoir 9 provided with a stirrer 16 and a cooling coil (not shown in the drawing). Transport of the crystals is effected by a conveyor screw 8. Sufficient concentrated nitric acid is continuously supplied to the reservoir 9 to produce a nitrating acid, consisting of 35-50 parts of S03, 65-50 parts of HNOs and 0-2% of H20. Part of this nitrating acid is supplied as N205 make-up to the buffer tank 2, via pump 10. The reservoir 9 is provided with a discharge conduit 17 through which the remaining amounts of nitrating acid, it any, may be drained.

A specific example illustrating operation of the process of the invention is now presented.

Per hour 100 m5 of contact gas containing 7% by volume of S03 was introduced into the column; of this amount per hour 200 kg. of 502 were absorbed in an absorption liquid composed of 70% of S03, 29% of HNOs and 1% of H20, while e. g., 4000 kg. per hour were circulated by pumping via reservoir 2 and pump 3; 25 kg. of non-absorbed S03 were discharged via discharge opening 5, together with the residual gases.

An amount of 240 kg. per hour of N205 make-up liquid was furthermore supplied to reservoir 2 in a continuous current (composition: 45% of S03, 53% of HNOs, 2% of H20), while via discharge conduit 6 440 kg. per hour of melted substance was conveyed to the drum cooler. 155 kg; per hour of the crystallized product obtained from the drum cooler was introduced into reservoir 9, which at the start of the process was filled with make-up liquid of the composition given above. Moreover, an amount of 85 kg. per hour of 97% HNOs was supplied to reservoir 9.

The total amount of crystalline product produced per 4 hour was 285 kg., recovered from 100 tn. of contact gas and kg. of 97% HNOs.

The total heat of reaction liberated in the reaction between this S03 and nitric acid is, in the process according to the invention, removed at four points, viz. on the drum cooler 7, in the reservoirs 2 and 9 and by radiation in the absorption column 1.

Thus the heat of reaction is removed quickly andthe process is exceptionally easy to regulate. Decomposition of nitric acid as a result of excessive temperatures is excluded.

The method described above is also suited for the production of a high-grade nitrating acid consisting of 35-50 parts of S03 and 65-50 parts of HNOa; e. g., when, operating under the same conditions all of the crystals removed from the drum cooler were dissolved in reservoir 9, while adding at the same time 243 kg. of 97% HNO: per hour, it was possible to remove from reservoir 9, 443 kg. of high-grade nitrating acid per hour, said acid consisting of 45% of S02, 53% of HNOs and 2% of H20.

Furthermore, it is also possible to dissolve the solid crystals in nitric acid in a separate reservoir 11; in this manner nitrating acids of widely varying compositions may be obtained without great rises in temperature occurring.

In this way a high-grade nitrating acid was obtained by dissolving 80 kg. of crystalline (N02+) (I-ISzOr") in 25 kg. of 98% HNOs. The resulting nitrating acid contained 54% of S03, 45 of HNOs and 1% of H20; during the dissolution the temperature rose from 20 to 40 C.

Likewise, a nitrating acid consisting of 66% of H2SO4 and 34% of HNOa was e. g. obtained by dissolving 100 kg. of crystalline (NO2+) (HSz0'z-) in 31 kg. of 53% nitric acid. During this dissolution the temperature rose to 70 C., which temperature could easily be kept below 50 C. by cooling.

If the same nitrating acid were to be prepared by gradual addition of oleum to concentrated nitric acid, e. g., 60 parts of 60% oleum to 40 parts by Weight of 86% HNOs, a very strong rise in temperature occurs immediately, evolving S03 and nitric fumes and effecting a considerable loss of nitric acid by decomposition.

As understood by those skilled in the art, satisfactory common nitrating acids consisting of a mixture of sull uric acid and nitric acid and containing apart from water from 80-60% H2504 and from 20-40% HNO: and free S03 containing nitrating acids consisting apart from 0-5% of water of a mixture of 60-35 parts of S03 and 40-65 parts of HNOs can be prepared and the term nitrating acid as used herein is intended to mean acids having the compositions encompassed above.

It will be appreciated that the proportions of crystalline (Noz (HSzOF) to concentrated nitric acid used according to the present invention can be widely varied depending on such factors as the concentration of the nitric acid employed and the composition of the nitrating acid desired. However, generally speaking, it can be stated that from 100 to 400 kg. of (N0z+) (I-ISzOq") per 100 kg. of concentrated nitric acid of 98% can be used satisfactorily.

If a nitric acid of 40-60% HNOs is used 400 to 700 kg. of Non) (HSzOw) per 100 kg. nitric acid has to be used in order to obtain a satisfactorily nitrating acid.

I claim:

1. In the process of producing solid crystalline (N02 (HS207 which comprises supplying to an absorption liquid of about the same composition as the desired product maintained at a temperature above 100 C. an S03- containing gas and a liquid-containing nitric acid, the proportions of the supplied ingredients being adjusted to maintain a molar ratio of SOsZNzOs of about 4, withdrawing a portion of the absorbing liquid in an amount corresponding to the amount of supplied ingredients, and cooling the withdrawn liquid to crystallize (NOifl) (HSsO'r'), the improvement which comprises employing as the liquid containing nitric acid which is supplied to the absorption liquid a mixture produced by dissolving crystalline (N0z+) (HSzOF) in concentrated nitric acid, said mixture consisting essentially of from to parts of S03, to 50 parts of HNO3 together with up to 2% Water.

2. A process as recited in claim 1 in which the crystalline (NOz (HSzO'F) produced by cooling and crystallizing the withdrawn liquid is employed as a part of the nitric acid containing liquid supplied to the absorption liquid.

3. A process as recited in claim 1 in which the S03 containing gas contains about 6-8% by volume of S03.

4. A process for the production of nitrating acid which comprises continuously absorbing S03 in a liquid mixture of about the same composition as the product (N0z+) (HS207 maintained at a temperature over 100 C., continuously supplying N205 make-up liquid consisting essentially from 35 to 50 parts S03 to 65 to 50 parts HNOs together with up to 2% by weight of the liquid mixture of water, the molar ratio of S03 absorbed to N205 supplied being about 4, continuously withdrawing a portion of the absorbed liquid in an amount corresponding to the amount of absorbed S03 and supplied N205, cooling said withdrawn liquid to crystallize (NO2+) (HSzOF) dissolving a portion of said crystals in concentrated nitric acid to produce the said N205 make-up liquid, and dissolving the remaining (N02+) (HS207 in concentrated nitric acid to produce nitrating acid.

5. A process according to claim 1 wherein the dissolution is carried out at a temperature not above about C.

References Cited in the file of this patent UNITED STATES PATENTS 884,035 Nield Apr. 7, 1908 1,047,576 Schultze Dec. 17, 1912 FOREIGN PATENTS 524,531 Great Britain Aug. 8, 1940 OTHER REFERENCES Heertjes et al., in Chemical Abstracts, vol. 44, col. 8809(g) (1950). 

1. IN THE PROCESS OF PRODUCING SOLID CRYSTALLINE (NO2+) (SH2O7-) WHICH COMPRISES SUPPLYING TO AN ABOSRPTION LIQUID OF ABOUT THE SAME COMPOSITION AS THE DESIRED PRODUCT MAINTAINED AT A TEMPERATURE ABOVE 100* C. AN SO3CONTAINING GAS AND A LIQUID-CONTAINING NITRIC ACID, THE PROPORTIONS OF THE SUPPLIED INGREDIENTS BEING ADJUSTED TO MAINTAIN A MOLAR RAIO OF SO3:N2O5 OF ABOUT 4, WITHDRAWING A PORTION OF THE ABSORBING LIQUID IN AN AMOUNT CORRESPONDING TO THE AMOUNT OF SUPPLIED INGREDIENTS, AND COOLING THE WITHDRAWN LIQUID TO CRYSTALLIZE (NO2+) (HS2O7-), THE IMPROVEMENT WHICH COMPRISES EMPLOLYING AS THE LIQUID CONTAINING NITRIC ACID WHICH IS SUPPLIED TO THE ABOSRPTION LIQUID A MIXTURE PRODUCED BY DISSOLVING CRYSTALLINE (NO2+) (HS2O7-) IN CONCENTRATED NITRIC ACID, SAID MIXTURE CONSISTING ESSENTIALLY OF FROM 35 TO 50 PARTS OF SO3, 65 TO 50 PARTS OF HNO3 TOGETHER WITH UP TO 2% WATER. 